Sets a target name that the user can then use to configure various
‘with’ options with on the command line by using that name. For
example, if the target is set to “foo”, then the user could use the
–with-foo-dir command line option.

You may pass along additional ‘include’ or ‘lib’ defaults via the
idefault and ldefault parameters, respectively.

Note that dir_config only adds to the
list of places to search for libraries and include files. It does not link
the libraries into your application.